With respect to an optical wave guide mechanism, semiconductor laser devices are classified into the gain guided type and the refractive index guided type. The former gain guided type is met with various application inconveniences since this type is unstable as to a transverse mode and astigmatism is large which indicates displacements of beam waists (i.e., a position at which the beam width is minimum) in directions parallel and perpendicular to the junction. On the other hand, the latter refractive index guided type is advantageous due to stability as to transverse mode and small astigmatism.
An example of a semiconductor laser of the refractive index guided type in a BH (Buried Heteroastructure) laser. A BH laser, with an active layer buried in a material having a low refractive index, is advantageous in that the BH laser is completely of the refractive index guided type, has a small threshold current Ith, oscillates in a basic transverse mode, and has small astigmatism. On the other hand, a BH laser is not suitable as a high-output laser, since damage or impurities introduced into the active layer during processing of the active layer functions as a non-emission recombination center.
Furthermore, as a laser of a type which creates a refractive index difference in the vicinity of an active layer, creates a refractive index distribution and confines a transverse mode, there are a CSP (Channeled Substrate Planar) laser and a VSIS laser. Since in the vicinity of an active layer in such a laser, current blocking layers having a large absorption coefficient of laser light are formed so that it is possible to control a relatively small refractive index difference, thus obtaining basic mods oscillations even with a large stripe width. However, this absorption serves as an internal loss, and therefore, the threshold current Ith becomes large and a differential efficiency becomes small.
To solve the problems above described, a semiconductor laser of the refractive index guided type with a small loss which uses a current blocking layer which is not absorbent and have a low refractive index has been proposed (IEEE Journal of Quantum Electronics, Vo.29, No.6, p.1889-1894. (1993)).
FIG. 8(a) is a structure view showing an example of the semiconductor laser of the refractive index guided type with a small loss and uses a non-absorbent current blocking layer mentioned above, while FIG. 8(b) is a graph showing a guided mode of the semiconductor laser. In FIG. 8(a), on a buffer layer 8 of GaAs, a clad layer 7 of AlGaAs, a wave guide layer 6 of AlGaAs, an active layer 5 of GaAs, a wave guide layer 4 of AlGAs, a clad layer 2 of AlGaAs, and a cap layer 1 of GaAs are formed in this order, and current blocking layers 3 which have higher Al-composition and a lower refractive index than that of the clad layer is formed in the clad layer to sandwich a stripe-shaped active region 10. This creates a refractive index difference between the active region 10 and buried regions 9 in which the current blocking layers 3 are formed, whereby a refractive index guide structure is obtained.
This structure is based on an SCH (Separate Confinement Heterostructure) structure which is generally used as a high-output semiconductor laser, where by burying the current blocking layers 3 having higher Al-composition and a lower refractive index than that of the clad layer in the vicinity of the active layer 5 to create a refractive index distribution in a transverse direction, the buried current blocking layers 3 do not absorb laser light to make an internal loss small and a basic transverse mode oscillations are realized up to high optical outputs.
However, it is known that the semiconductor laser of the refractive index guided type shown in FIGS. 8(a) and 8(b) has a very small production margin and a low production yield. In other words, to ensure stable laser oscillations up to relatively high optical outputs, an effective refractive index difference .DELTA.Neff between an action region and burled regions which are requirements in a refractive index guide structure has been discussed in various types of literature. For instance, .DELTA.Neff.gtoreq.5.times.10.sup.-3 in the reference mentioned above, while .DELTA.Neff.gtoreq.4.times.10.sup.-3 in other reference (i.e., Japanese Patent Examined Publication (koukoku) No. 6-36456(1994)). Thus, in order to ensure a refractive index difference of a certain value or more, it is necessary to form current blocking layers having a sufficiently low refractive index sufficiently close to an active layer.
Still, it is very difficult to control an effective refractive index using current blocking layers of a low refractive index in the conventional SCH structure. A first reason is that if a wave guide path is too asymmetric due to forming current blocking layers, a normalized frequency is cut off with respect to a basic mode as well and a guide structure is accordingly destroyed, and hence, an internal loss is increased and a differential efficiency is deteriorated. A second reason is that it is difficult to control a position of the current blocking layers. While current blocking layers are formed by etching in most cases, since it is very difficult to process during etching at an accuracy of 0.1 .mu.m or lower, there are limits of position and layer thickness in the current blocking layers. A third reason is that if AlGaAs is used for the current blocking layer, for instance, since it is necessary to suppress Al-composition as low as possible in the current blocking layer, in order to prevent oxidation in the vicinity of the current blocking layers and deterioration in crystallization during the processing, it is difficult to create a sufficient refractive index difference (Al-composition difference) between a clad layer and current blocking layers in the conventional structure.
After all, under a few restrictions regarding a current blocking layer, it is very difficult to form a refractive index guide structure using current blocking layers having a low refractive index in the conventional SCH structure, and a production margin is small.